The RCA Standard Clean developed by Werner Kern and other RCA scientists in the late 60's is extremely effective in removing contamination from silicon surfaces of semiconductor wafers and is today and has been the defacto industry standard for more than 30 years.
The rapid progress in the semiconductor industry, due in large part to the effectiveness of RCA clean (i.e., SC1/SC2), is described in detail in Werner Kern's 1993 book “Handbook of Semiconductor Wafer Cleaning” (680 pages).
The industry still relies on this hydrogen peroxide-based wet-chemical process where wafers are immersed in several chemicals sequentially to remove particles, metallics, organics, and native oxides.
During the last two decades, the standard wafer cleaning processes used in the fabrication plants included both megasonic transducer means and RCA-type cleaning solutions. The preferred megasonic cleaning system after the year 1990 employed direct-coupled transducer means (See 1989 U.S. Pat. No. 4,804,007, Verteq, Inc.). Such means was capable of providing at least 3 watts of RF power per square centimeter of the transmitting surface.
Later, an improved transducer means developed by PCT Systems, Inc., was able to provide much more power by avoiding or reducing unwanted interference between different sonic waves as described, for example, in U.S. Pat. No. 6,098,643. Such high-power transducer means, when operated at high energy levels above 5 watts per square centimeter, were more effective in removing particulates and greatly reduced the time required for satisfactory wafer cleaning. However, such megasonic transducer means was inefficient and ineffective in removing particles with a size smaller than 0.12 micron.
The sub 0.1-micron killer particles left on the wafer surface after cleaning presented a major, major problem. During the last decade, in the fabrication of advanced devices having a line width or feature size of 0.25 micron, the killer particles in the size range of from 0.05 to 0.07 micron, for example, that could not be eliminated by megasonic means, were particularly troublesome. There were a number of possible ways to reduce the number of such killer particles as by repeated treatment of the wafer surface with aqueous ammoniacal or SC-1 solutions, repeated or multiple DI water rinses, longer chemical treatment sequences, repeated treatment with corrosive diluted HF acid solutions, or other more sophisticated techniques.
The use of sophisticated chemical treatments or other special measures combined with the best available megasonic means was highly advantageous but not an ideal or complete solution to the particulate contamination problem. It made possible the commercial manufacture of advanced devices with a line width or feature size of 0.18 micron. The semiconductor industry was, of course, aware of the major shortcomings of megasonic wet wafer cleaning systems and their inability to eliminate sub 0.1-micron particles. It was manifest to everyone that the known megasonic wafer cleaning systems were deficient and inadequate for manufacture of the more advanced microelectronic devices, such as those having a line width or feature size of 0.10 micron or less and also that extensive research would be needed to solve the seemingly impossible particle contamination problem.
In recent years, researchers came to realize that the use of excessive megasonic energy during the fabrication process was risky and could degrade the microcircuits and cause them to be unreliable, and that serious defects would often be undetectable. In the case of more advanced MOS devices having ultrathin gate oxide layers with a thickness of from 10 to 20 angstroms, the impact of high-energy megasonic pressure waves would be intolerable. It therefore, would be preferable to operate at a safe or limited power level, no greater than about 3 watts per square centimeter, to minimize the risks in spite of the fact that the lowered power substantially reduced the effectiveness of the pressure waves and made it impossible to eliminate the harmful sub 0.1-micron contaminant particles.
The particulate contamination problem is a crucial one that must be solved soon in order to permit manufacture of advanced new generation devices with line widths or feature sizes, such as 0.05 to 0.10 micron, or gate oxide thicknesses of 10 to 20 angstroms. For that reason there has been an intense and frantic research effort for the last 5 years to solve the problem. It has so far been unsuccessful. The foremost experts in the field concluded that megasonic systems or other wet wafer cleaning systems were not promising, that they provided no practical way to remove or eliminate sub 0.1-micron killer particles, and that the semi-conductor industry would have to develop more sophisticated dry wafer cleaning processes.
Heretofore the semiconductor industry believed that wet cleaning methods would never be effective in removing adhered colloidal-size contaminant particles. For the last 30 years the most competent scientists have been convinced that the primary force binding a colloidal-size particle to a wafer surface is van der Waals attraction which is universal and dominating when separation distances between a particle and a surface are extremely small. The forces of attraction increase as the particle size decreases so that it appears virtually impossible to overcome the van der Waals forces when the particle size is 0.01 micron or less.
On this basis the best scientific minds grappling with the problem including foremost experts, such as Werner Kern, concluded that wet cleaning processes could not provide a satisfactory way to remove colloidal-size particles when manufacturing the most advanced microchips and that the only real hope for success was a breakthrough or major improvement in dry wafer cleaning technology.